1. Field of the Invention
The invention relates generally to position sensing methods and control systems (e.g., servo control systems), and more particularly to methods and systems for sensing the position of a conductive material with electromagnetic or capacitive sensors.
2. Description of the Related Art
Positioning sensors and servo systems are employed in many contexts requiring accurate positioning of device components. For instance, head positioning servo systems may be employed in media drives for accurately positioning a read and/or write head over a selected data track of a storage medium. Many storage systems and media, e.g., optical or magnetic, may employ various positioning sensors and servo systems to increase data recording and retrieval processes. For illustrative purposes only, magnetic tape storage systems and media are described herein.
Magnetic tape-recording remains a viable solution for storage of large amounts of data. Conventionally, at least two approaches are employed for recording digital information onto magnetic recording tape. One approach calls for moving a magnetic tape past a rotating head structure that reads and writes user information from discontinuous transverse tracks. Interactive servo systems are typically employed to synchronize rotation of the head structure with travel of the tape. Another approach is to draw the tape across a non-rotating head at a considerable linear velocity. This approach is sometimes referred to as linear “streaming” tape recording and playback.
In the case of linear tape recording a popular trend is toward multi head, multi-channel fixed head structures with smaller recording gaps and narrower data track widths so that many linear data tracks may be achieved on a tape medium of a predetermined width, such as one-half inch width tape. To increase the storage density for a given cartridge size the bits on the tape may be written to smaller areas and on a plurality of parallel longitudinal tracks. As more tracks are recorded on a tape, each track becomes increasingly narrow. The tape therefore becomes more susceptible to errors caused from the tape shifting up or down (called lateral tape motion or “LTM”) in a direction perpendicular to the tape travel path as the tape passes by the magnetic head. In order to maintain proper alignment of the head with the data tracks on the tape, the tape is generally mechanically constrained to minimize LTM and data retrieval errors.
Lateral tape motion is generally defined as the undesirable movement (in-plane) of the tape perpendicular to its prescribed longitudinal direction of motion past a read/write head. Lateral tape motion and the ability to compensate for lateral tape motion is a major limiting factor in determining the minimum width of a track and the minimum spacing between tracks on the tape. Thus, as lateral tape motion is reduced, more tracks may be stored on the tape and the tape data storage capacity increases accordingly.
Tape substrates are also being made thinner to increase the cartridge volumetric data density. Thinner tape substrates allow a longer tape to be contained within the same size diameter reel packages, thereby increasing the data storage of the cartridge. Thinner tapes, however, are generally less rigid making them more susceptible to lateral tape motion.
One approach to minimize lateral tape motion tracking errors is to provide a multi-roller tape guide structure, such as the type described in commonly assigned U.S. Pat. No. 5,414,585, entitled “Rotating Tape Edge Guide,” the disclosure thereof being incorporated herein by reference in its entirety. Such an approach has provided a viable “open loop” solution to lateral tape motion, i.e., control of lateral tape motion without the use of feedback. With the advent of new head technologies, such as magneto-resistive read heads, and new higher coercivity recording media, data track widths have become very small, and many additional data tracks may be defined on the tape. Unfortunately, lateral tape motion remains as a limiting factor, and at certain data track width dimensions and data track densities, it is mechanically prohibitive to reduce LTM to follow the tape accurately enough to provide reliable performance during reading and writing operations.
Several “closed loop” methods have been developed to minimize LTM tracking errors, including the use of magnetically recorded or optically detectable servo tracks positioned on a magnetic tape. The servo tracks allow for increased tracking abilities, effectively reducing LTM through servo track feedback mechanisms and the like. These methods, however, have not been able to keep pace with the increased data capacity desired for magnetic tape storage media.
Accordingly, new methods and systems for sensing the relative position of a conductive material are desired. For example, increased tracking sensitivity to detect and correct for LTM and decrease tracking errors allowing for increased data storage capabilities of the storage media is desired.